O que é Bid–ask spread and mid price?

Example 1.3 (The dentist’s order)

The market is $49.99$ bid, $50.01$ ask: $s = \$0.02$, $m = \$50.00$, and the spread is $0.02/50.00 = 4\,\mathrm{bp}$ of the price. The dentist’s market order buys at the ask and pays the half-spread, $0.01 a share, that is $1.00 on her hundred shares. That dollar is the market maker’s gross revenue on the trade — before the price has had a chance to move.